Loans - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Loans 90 days or more past due and accruing interest	$ 0		$ 0		$ 0
Internal risk ratings of loans	327,512,000		327,512,000		372,589,000
Additional committed impared loans	0		0		0
Loans modified as troubled debt restructurings	0	$ 0	0	$ 0
Troubled debt restructurings within past twelve months for which there was a default	0	$ 0	0	$ 0
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Internal risk ratings of loans	$ 0		0		0
Other Financial Institutions [Member]
Loans and Leases Receivable Disclosure [Line Items]
Participation loans transferred			$ 87,121,000		$ 61,328,000

Additional committed impaired loans.

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- Definition

Participation loans transferred.

+ References

No definition available.

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- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
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-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

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- Definition

Amount of financing receivable 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

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